Semiconductor nanocrystals whose dimensions are comparable to the bulk exciton diameter show quantum confinement effects. This is seen most clearly in the optical spectra which shift to blue wavelengths as the size of the crystal is reduced.
Semiconductor nanocrystals made from a wide range of materials have been studied including many II-VI and III-V semiconductors. In addition to spherical nanocrystals, rod-, arrow-, teardrop- and tetrapod-shaped nanocrystals [Alivisatos et al., J. Am. Chem. Soc, 2000, 122, 12700; WO03054953] and core-shell structures [Bawendi, J. Phys. Chem. B, 1997, 101, 9463; Li and Reiss, J. Am. Chem. Soc., 2008, 130, 11588] have also been prepared. To control the size and shape of such nanocrystals their synthesis is generally performed in the presence of one or more capping agents (sometime called surfactants or coordinating solvents). Such capping agents control the growth of the nanocrystals and also increase the strength of the light emission though the passivation of surface states. A wide range of capping agents have been employed including phosphines [Bawendi et. al., J. Am. Chem. Soc., 1993, 115, 8706], phosphine oxides [Peng et. al., J. Am. Chem. Soc., 2002, 124, 2049], amines [Peng et. al., J. Am. Chem. Soc., 2002, 124, 2049], fatty acids [Battaglia and Peng, Nano Lett., 2002, 2, 1027; Peng et. al., J. Am. Chem. Soc., 2002, 124, 2049], thiols [Li and Reiss, J. Am. Chem. Soc., 2008, 130, 11588] and more exotic capping agents such a metal fatty acid complexes [Nann et. al., J. Mater. Chem., 2008, 18, 2653].
Methods to prepare semiconductor nanocrystals include solvothermal reactions [Gillan et. al., J. Mater. Chem., 2006, 38, 3774], hot injection methods [Battaglia and Peng, Nano Lett., 2002, 2, 1027], simple heating processes [Van Patten et. al., Chem. Mater., 2006, 18, 3915], continuous flow reactions [US2006087048] and microwave assisted synthesis [Strouse et. al., J. Am. Chem. Soc., 2005, 127, 15791].
One of the most interesting classes of semiconductors is the III-nitrides, such as AlN, GaN, InN and their respective alloys. These are used for the manufacture of blue light-emitting diodes, laser diodes and power electronic devices. Nitrides are also chemically inert, are resistant to radiation, and have large breakdown fields, high thermal conductivities and large high-field electron drift mobilities, making them ideal for high-power applications in caustic environments [Neumayer at. al., Chem., Mater., 1996, 8, 25]. The band gaps of aluminium nitride (6.2 eV), gallium nitride (3.5 eV) and Indium nitride (0.7 eV) [Gillan et. al., J. Mater. Chem., 2006, 38, 3774] mean that nitrides span much of the ultraviolet, visible and infrared regions of the electromagnetic spectrum. The fact that alloys of these materials have direct optical band gaps over this range makes these very significant for optical devices. In the case of nanocrystals based on III-nitride semiconductors, tuning the band gap through alloying and quantum confinement effects opens up the possibility of making unique nanocrystalline phosphors spanning a wide region of the electromagnetic spectrum. However, to date, routes to fabricate nitride nanocrystals have resulted in only weakly emissive materials and have had poor control over the size of the nanocrystals produced.
Nanocrystalline indium nitride and indium gallium nitride have been prepared from the solvothermal reaction of metal halides with sodium azide [Gillan et. al., J. Mater. Chem., 2006, 38, 3774]. No emission spectra of the material were presented although some images from a fluorescence microscope were included. Nanocrystalline indium nitride has also been prepared from the solvothermal reaction of indium iodide with sodium amide [Xie et. al., New. J. Chem., 2005, 29, 1610]. In this work indium nitride nanocrystals were prepared and emission spectra are reported but no indication as to the intensity of the emission, such as a photoluminescent quantum yield, is reported. Other workers have attempted to prepare nitride nanocrystals in the presence of capping agents, but strong emission of light has never been reported in nitride nanocrystals prepared in these ways. [Mićić et. al., Appl. Phys. Lett., 1999, 74, 478; Van Patten et. al., Chem. Mater., 2006, 18, 3915; Cole-Hamilton et. al., J. Mater. Chem., 2004, 14, 3124; Rao et. al., Small, 2005, 1, 91].
US 2008/0173845 proposes a method of producing, with high synthesis yield, a coated nanocrystalline phosphor by heating a mixed solution containing a core of a group IIIB nitride semiconductor, a nitrogen-containing compound, a group IIIB element-containing compound and modified organic materials. The document states that the method leads to a nanocrystal with improved luminous efficiency, but no values for the photoluminescence quantum yield are given.
US 2006/000119 and US2006/0014040 disclose a semiconductor nano crystal complex in which a metal layer is formed on the outer surface of a semiconductor nanocrystal core.
US 2006/240227 discloses various semiconductor nanocrystals. The described examples relate primarily to CdSe or CdSe/ZnS structures. The document refers to a quantum yield for fluorescence of 45% and to a quantum yield in photoluminescence of 40-90%, in connection with CdSe/ZnS structures. Methods similar to those proposed in this document have been applied to nitride systems and have been found not to lead to emissive materials.
WO 01/52741 proposes a nanocrystal intended to allow in vivo glucose measurement, by illuminating the nanocrystal and measuring the emitted light output. It does not give any values for the PLQY of the nanocrystals.
US 2007/0111488 proposes a method for the fabrication of non-polar indium gallium nitride films.
WO 2007/020416 and WO2009/040553 (which was not published until April 2009) relate to the fabrication of core-shell structures, in particular how the core and shell are fabricated. They do not primarily relate to nitrides. They propose the use of organic capping agents to cap surface atoms which are not fully co-ordinated.
WO 2008/094292 relates to manufacture of a semiconductor nanostructure, including core-shell structures. It proposes using a few specified chelating ligand solutions such as, for example, TOPO (trioctylphosphine oxide) and TOP (trioctylphosphine).
WO2009/040553, which was not published until April 2009, relates to the fabrication of core-shell structures in general, in particular how the core and shell are fabricated. It proposes use of standard organic molecules as capping agents. It also proposes formation of a metal oxide shell, using a metal carboxylate as a precursor.
WO 2007/020416 contains generally similar teaching to WO2009/040553 (with both documents being by the same applicant). It again proposes use of standard organic molecules as capping agents.
WO 2008/094292 relates to manufacture of a semiconductor nanostructure, including core-shell structures. It proposes growth in a “chelating ligand solution”, and proposes various lyophilic surfactant molecules for this.